Endothelin-like immunoreactivity (ET-IR) was detected immunocytochemically in glandular epithelium and vascular endothelium of human endometrium and myometrium. Primary antibody was raised in rabbits against the carboxy-terminal heptapeptide of endothelin 1 (ET-1), ET-1(15-21), and compared with antibodies raised against the cyclized amino-terminal, ET-1(2-13), and commercially obtained antibodies against the whole ET-1 or ET-3 molecule. Binding was visualized using the peroxidase technique in sections counter-stained with haemalum. Staining was seen in each of 15 sections from eight women in the proliferative (five) or secretory (three) phase of the cycle. Intense staining was present in the cytoplasm of endometrial glands and vascular endothelium, and was greatest at the endometrial-myometrial junction. The pattern of staining was similar with all primary antibodies tested. The demonstration of ET-IR in endometrium suggests that the endothelins may play a role in control of the uterine vascular bed.